1. Field of the Invention
The present invention relates generally to signal processing using partial response signaling and maximum likelihood estimation, and more specifically to processing of high-density playback signals such as those recorded on optical discs as a series of microscopic pin or those recorded on magnetic mediums using vertical recording techniques.
2. Description of the Related Art
In playback systems for optical or magnetic discs, equalizers are usually employed to compensate for intersymbol interference to improve the quality of high-density signals extracted from a recording disc. In the case of optical discs, microscopic pits produced by a laser beam are closely spaced apart. When a portion of the disc is irradiated continuously by the beam to form a long pit, the recording level is not necessarily uniform across the length of the pit due to unequal distribution of optical energies over the length of the pit, and the amplitude a signal extracted therefrom is not uniform. Since this nonuniformity has a nonlinear characteristic and cannot be compensated for by conventional transversal filter equalizers which are implemented with linear circuits and devices.
Another problem associated with the playback of high-density recorded signals is that; due to the mixed presence of long and short pits which are spaced at very short intervals, the playback signals from short pits have not sufficient energy to reach the saturation levels and their amplitudes vary in a range far short thereof, while the amplitude of those signals read from long pits varies in a range close to the saturation levels.
Partial response equalization is known for intersymbol interference problems. As typically represented by a PR(1,1) equalizer using three reference amplitudes -1, 0 and +1 as target values of equalization, the gain-versus-frequency characteristic of such equalizer produces an output having an emphasized gain at high frequencies. If a maximum likelihood estimation is used in combination with a PR(1,1) equalizer for reproducing high-density signals, the amplitude nonuniformity of readout signals and the characteristic high-frequency emphasis of the equalizer would produce a noise which seriously affects the estimation process.
Therefore, there is a need to improve the signal-to-noise ratio of a playback system for high density recorded signals.